Planning for disk replacement

[hangdog]

Hi,

I’m considering using TrueNAS as part of a project to replace an ageing mini-ITX box. I’m not familiar with TrueNAS, although I worked as a sysadmin before retirement.

The question I have is regarding providing a spare drive for a vdev, and is probably best asked by way of an example. Suppose I have two NVMe slots available for a mirror, and a SATA slot which I’m intending to use for an emergency spare. I buy 2 identical 2TB NVMe drives, and a 2TB SATA drive from a different manufacturer.

When I come to commission the drives I find that the SATA drive is marginally smaller in terms of available blocks than the NVMe drives.

Is it possible with TrueNAS to create a mirror with the NVMe drives which is compatible with using the very slightly smaller drive as a spare? In my previous life I would have used partitions for this, by my understanding of TrueNAS is that this isn’t how things are done. Have I misunderstood something?

Thanks for any info, and apologies if this has a very obvious answer.

[SmallBarky]

This may be what you are asking about. We used to have the buffer space, it disappeared and now should be back in Scale

[Accepted] Create 2 GiB buffer space when adding a disk

Problem/Justification When replacing a drive in a vdev, a new drive even from the same manufacturer may just be a hair too small to fit the vdev. Examples of users impacted by the recent change in SCALE 24.04.1 to remove the default 2 GiB swap partition, which acted as this buffer: Impact With a default buffer space of 2 GiB, a slightly-too-small replacement drive could still be added with a smaller or no buffer on that drive. FreeNAS, TrueNAS CORE and TrueNAS SCALE before 24.04.1 did t…

[hangdog]

That’s a fascinating thread, and answers my question exactly - thanks. I should be able to create (e.g.) a 2TB vdev under 25.04 and pretty much any 2TB disk should be capable of being substituted for any one of the vdev members.